[swarming] catch RPCFailedError in _get_memory_usage
[swarming] catch RPCFailedError in _get_memory_usage

It happens occasionally
http://shortn/_3f8DLsbWKU

It also may be related to DeadlineExceededError system.GetSystemStats?
http://shortn/_ivk8d2Mehb

Bug: 1027407
Change-Id: I0d6056fd7c72939c54764a5010c9b76585fbbd90
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1980373
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py